Chemical composition of fats

Although natural fats consist primarily of glycerides, they contain many other lipids in minor quantities. Corn oil, for example, may contain glycerides plus phospholipids, glycolipids, phosphoinositides (phospholipids containing inositol), many isomers of sitosterol and stigmasterol (plant steroids), several tocopherols (vitamin E), vitamin A, waxes, unsaturated hydrocarbons such as squalene, and dozens of carotenoids and chlorophyll compounds, as well as many products of decomposition, hydrolysis, oxidation, and polymerization of any of the natural constituents.

Fatty acids contribute from 94 to 96 percent of the total weight of various fats and oils. Because of their preponderant weight in the glyceride molecules and also because they comprise the reactive portion of the molecules, the fatty acids influence greatly both the physical and chemical character of glycerides. Fats vary widely in complexity; some contain only a few component acids, and at the other extreme more than 100 different fatty acids have been identified in butterfat, although many are present in only trace quantities. Most of the oils and fats are based on about a dozen fatty acids (see table). In considering the composition of a glyceride it is particularly important to distinguish between the saturated acids (acids containing only single bonds between carbon atoms, such as palmitic or stearic), with relatively high melting temperatures, and the unsaturated acids (acids with one or more pairs of carbon atoms joined by double bonds, such as oleic or linoleic), which are low melting and chemically much more reactive.

In the series of saturated acids, the melting point increases progressively from below room temperature for the acids of lower molecular weight to high melting solids for the longer chain acids. Unsaturated acids may contain up to six double bonds, and as unsaturation increases the melting points become lower. Glycerides based predominantly on unsaturated acids, such as soybean oil, are liquids; and glycerides containing a high proportion of saturated acids, such as beef tallow, are solids. The carbon atoms in fatty acids are arranged in straight chains, and the first site of unsaturation (double bond) in most of the unsaturated acids appears between the ninth and tenth carbon atoms, starting the counting from the terminal carboxyl group (see table). The specificity of location of unsaturation in fatty acids obtainable from both plant and animal sources suggests that all are formed by a common enzymatic dehydrogenation mechanism.

Since the glycerides, which make up 90 to 99 percent of most individual fats or oils of commerce, are esters formed by three fatty-acid molecules combining with one molecule of glycerol, they may differ not only in the fatty acids that they contain but also in the arrangement of the fatty-acid radicals on the glycerol portion. Simple triglycerides are those in which each molecule of glycerol is combined with three molecules of one acid—e.g., tripalmitin, C₃H₅(OCOC₁₅H₃₁)₃, the glyceryl ester of palmitic acid, C₁₅H₃₁COOH. Only a few of the glycerides occurring in nature are of the simple type; most are mixed triglycerides (i.e., one molecule of glycerol is combined with two or three different fatty acids). Thus stearodipalmitin, C₃H₅(OCOC₁₅H₃₁)₂(OCOC₁₇H₃₅), contains two palmitic acid radicals and one stearic acid radical. Similarly, oleopalmitostearin, C₃H₅(OCOC₁₅H₃₁)(OCOC₁₇H₃₃)(OCOC₁₇H₃₅), contains one radical each of oleic, palmitic, and stearic acids. Each mixed triglyceride containing three different acid radicals may exist in three different isomeric forms, because any of the three can be linked with the centre carbon of the glycerol molecule. A mixed triglyceride containing two radicals of the same acid and one radical of another acid has only two isomeric forms.

Monoglycerides and diglycerides are partial esters of glycerol and have one or two fatty-acid radicals, respectively. They are seldom found in natural fats except as the products of partial hydrolysis of triglycerides. They are easily prepared synthetically, however, and have important applications mainly because of their ability to aid in the formation and stabilization of emulsions. As constituents of shortening in baked products they increase product volumes, improve tenderness, and retard staling. They also have technical importance as intermediates in the manufacture of coatings and resins.

Physical and chemical properties

Fats (and oils) may be divided into animal and vegetable fats according to source. Further, they may be classified according to their degree of unsaturation as measured by their ability to absorb iodine at the double bonds. This degree of unsaturation determines to a large extent the ultimate use of the fat.

Liquid fats (i.e., vegetable and marine oils) have the highest degree of unsaturation, while solid fats (vegetable and animal fats) are highly saturated. Solid vegetable fats melting between 20 and 35 °C (68 and 95 °F) are found mainly in the kernels and seeds of tropical fruits. They have relatively low iodine values and consist of glycerides containing high percentages of such saturated acids as lauric, myristic, and palmitic. Fats from fruits of many members of the palm family, notably coconut and babassu oils, contain large amounts of combined lauric acid. Most animal fats are solid at ordinary temperatures; milk fats are usually characterized by the presence of short-chain carboxylic acids (butyric, caproic, and caprylic); and marine oils contain a large number of very long chain highly unsaturated acids containing up to six double bonds and up to 24 or even 26 carbon atoms.

Fats are practically insoluble in water and, with the exception of castor oil, are insoluble in cold alcohol and only sparingly soluble in hot alcohol. They are soluble in ether, carbon disulfide, chloroform, carbon tetrachloride, petroleum benzin, and benzene. Fats have no distinct melting points or solidifying points because they are such complex mixtures of glycerides, each of which has a different melting point. Glycerides, further, have several polymorphic forms with different melting or transition points.

Fats can be heated to between 200 and 250 °C (392 and 482 °F) without undergoing significant changes provided contact with air or oxygen is avoided. Above 300 °C (572 °F), fats may decompose, with the formation of acrolein (the decomposition product of glycerol), which imparts the characteristic pungent odour of burning fat. Hydrocarbons also may be formed at high temperatures.

Fats are hydrolyzed readily. This property is used extensively in the manufacture of soaps and in the preparation of fatty acids for industrial applications. Fats are hydrolyzed by treatment with water alone under high pressure (corresponding to a temperature of about 220 °C [428 °F]) or with water at lower pressures in the presence of caustic alkalies, alkaline-earth metal hydroxides, or basic metallic oxides that act as catalysts. Free fatty acids and glycerol are formed. If sufficient alkali is present to combine with the fatty acids, the corresponding salts (known popularly as soaps) of these acids are formed, such as the sodium salts (hard soap) or the potassium salts (soft soaps).